Neptune

Neptune is the eighth planet from the Sun and is the most distant planet from the Sun. This gas giant planet may have formed much closer to the Sun in early solar system history before migrating to its present position.

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Facts about Neptune

Neptune was not known to the ancients:
It is not visible to the naked eye and was first observed in 1846. Its position was determined using mathematical predictions. It was named after the Roman god of the sea.

Neptune spins on its axis very rapidly:
Its equatorial clouds take 18 hours to make one rotation. This is because Neptune is not solid body.

Neptune is the smallest of the ice giants:
Despite being smaller than Uranus, Neptune has a greater mass. Below its heavy atmosphere, Uranus is made of layers of hydrogen, helium, and methane gases. They enclose a layer of water, ammonia and methane ice. The inner core of the planet is made of rock.

The atmosphere of Neptune is made of hydrogen and helium, with some methane:
The methane absorbs red light, which makes the planet appear a lovely blue. High, thin clouds drift in the upper atmosphere.

Neptune has a very active climate:
Large storms whirl through its upper atmosphere, and high-speed winds track around the planet at up 600 meters per second. One of the largest storms ever seen was recorded in 1989. It was called the Great Dark Spot. It lasted about five years.

Neptune has a very thin collection of rings:
They are likely made up of ice particles mixed with dust grains and possibly coated with a carbon-based substance.

Neptune has 14 moons:
The most interesting moon is Triton, a frozen world that is spewing nitrogen ice and dust particles out from below its surface. It was likely captured by the gravitational pull of Neptune. It is probably the coldest world in the solar system.

Only one spacecraft has flown by Neptune:
In 1989, the Voyager 2 spacecraft swept past the planet. It returned the first close-up images of the Neptune system. The NASA/ESA Hubble Space Telescope has also studied this planet, as have a number of ground-based telescopes.

Neptune is the eighth and farthest planet from the Sun in the Solar System. It is the fourth-largest planet by diameter and the third-largest by mass. Among the gaseous planets in the Solar System, Neptune is the most dense. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth, and not as dense as Neptune orbits the Sun at an average distance of 30.1 astronomical units. Named after the Roman god of the sea, its astronomical symbol is ♆, a stylised version of the god Neptune's trident.

Neptune was the first and only planet found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitationalperturbation by an unknown planet. Neptune was subsequently observed on 23 September 1846 by Johann Galle within a degree of the position predicted by Urbain Le Verrier, and its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining 13 moons were located telescopically until the 20th century. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989.

Neptune is similar in composition to Uranus, and both have compositions that differ from those of the larger gas giants, Jupiter and Saturn. Neptune's atmosphere, like Jupiter's and Saturn's, is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen; it contains a higher proportion of "ices" such as water, ammonia, and methane. Astronomers sometimes categorise Uranus and Neptune as "ice giants" to emphasise this distinction.The interior of Neptune, like that of Uranus, is primarily composed of ices and rock.Perhaps the core has a solid surface, but the temperature would be thousands of degrees and the atmospheric pressure crushing.Traces of methane in the outermost regions in part account for the planet's blue appearance

Neptune's mass of 1.0243×1026 kg, is intermediate between Earth and the larger gas giants: it is 17 times that of Earth but just 1/19th that of Jupiter. Its surface gravity is surpassed only by Jupiter. Neptune's equatorial radius of 24,764 km is nearly four times that of Earth. Neptune and Uranus are often considered a subclass of gas giant termed "ice giants", due to their smaller size and higher concentrations of volatiles relative to Jupiter and Saturn. In the search for extrasolar planets Neptune has been used as a metonym: discovered bodies of similar mass are often referred to as "Neptunes", just as astronomers refer to various extra-solar bodies as "Jupiters".

Neptune's internal structure resembles that of Uranus. Its atmosphere forms about 5% to 10% of its mass and extends perhaps 10% to 20% of the way towards the core, where it reaches pressures of about 10 GPa, or about 100,000 times that of Earth's atmosphere. Increasing concentrations of methane, ammonia and water are found in the lower regions of the atmosphere.

The mantle is equivalent to 10 to 15 Earth masses and is rich in water, ammonia and methane.As is customary in planetary science, this mixture is referred to as icy even though it is a hot, dense fluid. This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean. The mantle may consist of a layer of ionic water in which the water molecules break down into a soup of hydrogen and oxygen ions, and deeper down superionic water in which the oxygen crystallises but the hydrogen ions float around freely within the oxygen lattice. At a depth of 7000 km, the conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones. Very-high-pressure experiments at the Lawrence Livermore National Laboratory suggest that the base of the mantle may comprise an ocean of liquid diamond, with floating solid 'diamond-bergs'.

The core of Neptune is composed of iron, nickel and silicates, with an interior model giving a mass about 1.2 times that of Earth. The pressure at the centre is 7 Mbar (700 GPa), about twice as high as that at the centre of Earth, and the temperature may be 5,400 K

Atmosphere

At high altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune's vivid azure differs from Uranus's milder cyan. Because Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour.

Neptune's atmosphere is subdivided into two main regions: the lower troposphere, where temperature decreases with altitude, and the stratosphere, where temperature increases with altitude. The boundary between the two, the tropopause, occurs at a pressure of 0.1 bars (10 kPa). The stratosphere then gives way to the thermosphere at a pressure lower than 10−5 to 10−4 microbars (1 to 10 Pa).The thermosphere gradually transitions to the exosphere.

Models suggest that Neptune's troposphere is banded by clouds of varying compositions depending on altitude. The upper-level clouds occur at pressures below one bar, where the temperature is suitable for methane to condense. For pressures between one and five bars (100 and 500 kPa), clouds of ammonia and hydrogen sulfide are believed to form. Above a pressure of five bars, the clouds may consist of ammonia, ammonium sulfide, hydrogen sulfide and water. Deeper clouds of water ice should be found at pressures of about 50 bars (5.0 MPa), where the temperature reaches 273 K (0 °C). Underneath, clouds of ammonia and hydrogen sulfide may be found.

High-altitude clouds on Neptune have been observed casting shadows on the opaque cloud deck below. There are also high-altitude cloud bands that wrap around the planet at constant latitude. These circumferential bands have widths of 50–150 km and lie about 50–110 km above the cloud deck. These altitudes are in the layer where weather occurs, the troposphere. Weather does not occur in the higher stratosphere or thermosphere. Unlike Uranus, Neptune's composition has a higher volume of ocean, whereas Uranus has a smaller mantle.

Neptune's spectra suggest that its lower stratosphere is hazy due to condensation of products of ultraviolet photolysis of methane, such as ethane and acetylene.The stratosphere is also home to trace amounts of carbon monoxide and hydrogen cyanide.The stratosphere of Neptune is warmer than that of Uranus due to the elevated concentration of hydrocarbons.

For reasons that remain obscure, the planet's thermosphere is at an anomalously high temperature of about 750 K. The planet is too far from the Sun for this heat to be generated by ultraviolet radiation. One candidate for a heating mechanism is atmospheric interaction with ions in the planet's magnetic field. Other candidates are gravity waves from the interior that dissipate in the atmosphere. The thermosphere contains traces of carbon dioxide and water, which may have been deposited from external sources such as meteorites and dust.

Neptune's weather is characterised by extremely dynamic storm systems, with winds reaching speeds of almost 600 m/s (2,200 km/h; 1,300 mph)—nearly reaching supersonic flow. More typically, by tracking the motion of persistent clouds, wind speeds have been shown to vary from 20 m/s in the easterly direction to 325 m/s westward.At the cloud tops, the prevailing winds range in speed from 400 m/s along the equator to 250 m/s at the poles.Most of the winds on Neptune move in a direction opposite the planet's rotation.The general pattern of winds showed prograde rotation at high latitudes vs. retrograde rotation at lower latitudes. The difference in flow direction is believed to be a "skin effect" and not due to any deeper atmospheric processes.At 70° S latitude, a high-speed jet travels at a speed of 300 m/s.

Neptune differs from Uranus in its typical level of meteorological activity. Voyager 2 observed weather phenomena on Neptune during its 1989 fly-by, but no comparable phenomena on Uranus during its 1986 fly-by.

The abundance of methane, ethane and ethyne at Neptune's equator is 10–100 times greater than at the poles. This is interpreted as evidence for upwelling at the equator and subsidence near the poles.

In 2007, it was discovered that the upper troposphere of Neptune's south pole was about 10 K warmer than the rest of Neptune, which averages approximately 73 K (−200 °C). The temperature differential is enough to let methane, which elsewhere is frozen in the troposphere, escape into the stratosphere near the pole.The relative "hot spot" is due to Neptune's axial tilt, which has exposed the south pole to the Sun for the last quarter of Neptune's year, or roughly 40 Earth years. As Neptune slowly moves towards the opposite side of the Sun, the south pole will be darkened and the north pole illuminated, causing the methane release to shift to the north pole.

Because of seasonal changes, the cloud bands in the southern hemisphere of Neptune have been observed to increase in size and albedo. This trend was first seen in 1980 and is expected to last until about 2020. The long orbital period of Neptune results in seasons lasting forty years

Voyager 2 is the only spacecraft that has visited Neptune. The spacecraft's closest approach to the planet occurred on 25 August 1989. Because this was the last major planet the spacecraft could visit, it was decided to make a close flyby of the moon Triton, regardless of the consequences to the trajectory, similarly to what was done for Voyager 1's encounter with Saturn and its moon Titan. The images relayed back to Earth from Voyager 2 became the basis of a 1989 PBS all-night program, Neptune All Night.

During the encounter, signals from the spacecraft required 246 minutes to reach Earth. Hence, for the most part, the Voyager 2 mission relied on preloaded commands for the Neptune encounter. The spacecraft performed a near-encounter with the moon Nereid before it came within 4400 km of Neptune's atmosphere on 25 August, then passed close to the planet's largest moon Triton later the same day.

The spacecraft verified the existence of a magnetic field surrounding the planet and discovered that the field was offset from the centre and tilted in a manner similar to the field around Uranus. The question of the planet's rotation period was settled using measurements of radio emissions. Voyager 2 also showed that Neptune had a surprisingly active weather system. Six new moons were discovered, and the planet was shown to have more than one ring.

In 2003, there was a proposal in NASA's "Vision Missions Studies" for a "Neptune Orbiter with Probes" mission that does Cassini-level science. The work is being done in conjunction with JPL and the California Institute of Technology.Another, more recent proposal was for Argo, a flyby spacecraft that would visit Jupiter, Saturn, Neptune, and a Kuiper belt object. However, the focus would be on Neptune and its largest moon Triton to help plug a predicted 50-year gap in exploration of the system. New Horizons 2 might have also done a flyby.